Chimera: A single organism that is composed of cells of distinct DNA.
In Greek mythology, the Chimera was a fire-breathing lion-creature with a goat’s head fused on its back and a snake’s head for a tail. Chimerism has major presences in fantasy, science fiction, and pop-culture, from the monsters found in the Power Rangers to the latest Disney movie. In the Fullmetal Alchemist anime series, the exploration and creation of human-animal chimeras was portrayed as taboo and controversial, but a subject of obsession for the purposes of restoration of bodily function or gain of abilities.
Natural human-humans
Human-human chimerism can happen naturally, like when cells from a fetus migrate into the body of a pregnant woman or when a fetus absorbs a twin. The latter case is how one baffled man had children with DNA from his unborn and unknown twin fraternal brother, becoming both father and uncle. Most chimeras don’t have their lives significantly affected because their physical differences are usually subtle, like somewhat non-symmetrical hair patterns or slightly different eye colors. Those that can be substantially affected throughout their lives may have significant differences in skin pigmentation or possess hermaphroditic traits. With the increase and popularization of DNA testing since the 1980s, a number of unfortunate chimera mix-ups have been recorded.
Artificial human-humans
A person can have multiple sets of DNA by surgical means, such as in an organ transplant. In this scenario, the DNA of the donor would persist in the transplant, while the patient-recipient would maintain their own DNA. A blood transfusion could produce a temporary type of chimerism, as the blood cells in the patient-recipient would contain the donor DNA — until the cells are naturally replaced over 4–6 weeks. Most people have donated blood (and DNA) at some point in their lives. About 50,000 units of blood components are transfused each day in the US, so such temporary hybridizations are fairly common.
In a bone marrow transplant, the patient-recipient’s body would begin to produce blood cells with the donor DNA. This is because bone marrow contains stem cells from the donor that develop into blood cells (with donor DNA). A Nevada leukemia patient received a bone marrow transplant to successfully treat his cancer. He became a chimera with his blood having the DNA of his donor. Later, His lips, cheeks, and tongue also found to contain DNA from his donor. But most unexpectedly, a few years later his semen was tested and showed to be entirely from his donor. This result could be unnerving to both donor and recipient, yet presumably the outcome is worth the extension of life. Bone marrow transplants can permanently affect the DNA in your body, thus make you ineligible for testing services like 23andMe.
Myth to Reality: Animal-Humans
There are already cases of temporary animal-human hybrids in clinical settings, such as heart valve replacement from pigs and cows. Pigs have been a logical choice in transplants into humans because of the similarity in size and structure of the tissues. But, the human body can reject the foreign pig tissue which makes the development of pig-human hybrid tissue desirable.
More recently, techniques are being developed to allow scientists to grow human organs in pig bodies. Such technology would be a breakthrough for organ transplant science and the organ shortage. But the devil is in the details, and the proposed method would require the transplant of human stem cells into a pig embryo, allowing the harvesting of pig-human organs from the resulting pig fetus.
CRISPR has been a major scientific advance, allowing scientists to remove the genetic material from the pig that initiates growth of the desired organ and allow the human stem cells to step into the gap. CRISPR, which has been the subject of scandal, may also be relied upon to resolve some ethical concerns — such as preventing the animal from developing human cognitive functions. Bioethics concerns remain, and in the US such research is not eligible for public funding.
The brilliant and controversial Carlos Izpisúa Belmonte is the leading pioneer in this field. Besides the groundbreaking work with pig-human chimeras, he and collaborators have been responsible for experiments that show anti-aging in mice and correcting hereditary mutations in human embryos. He recently created a monkey-human chimera in China, circumventing ethical restrictions in the US and Europe. Belmonte and his colleagues are certainly ones to watch in this fascinating field.
Some weird questions you might have…
Will we see the lion-goat-snake? Unlikely, there appear to be limitations on the “distance” between species that allows chimerism. Although, work by Steve Goldman of the University of Rochester Medical Center has shown the implant of human brain cells in mice brains.
Can animal-humans be created without sophisticated molecular biology (i.e., “naturally”)? This subject led to some scary web searches. Here is one safe-for-work story for the curious reader.
